Device with separation limiting standoff

ABSTRACT

An MEMS device, having two substantially parallel surfaces are separated by an initial distance. At least one of the surfaces includes a raised feature that limits the gap between the surfaces to less than the initial distance when an actuating voltage is applied. In some embodiments, the raised feature limits the gap to about 66% of the initial distance.

CROSS REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

STATEMENT REGARDING MICROFICHE APPENDIX

Not applicable.

BACKGROUND

This invention is directed to MEMS devices.

The parallel plates of an electrostatically actuated MEMS plate devicemay be pulled together by a voltage (V) applied across the plates. Thedisplacement of the plates is determined by a balance between theelectrostatic force and the restoring force of the springs that supportthe plates. It is well known that when increasing V, such that the gapbetween the plates is at or below 66% of the initial (V=0) separation,the plates' relative position becomes unstable, resulting in the runawayacceleration of the plates toward one another and ultimately impact.This impact can be harmful to the MEMS device, causing cold-welding,cracking, material transfer, coining, or embossing, all of which candecrease the reliability of the device.

Three methods have been used to mitigate the effects of runaway at the ⅓point of electrostatic actuation:

-   -   1) Charge control. At constant voltage the electrostatic force        increases roughly quadratically as the separation of the plates        is reduced, while the restoring spring force increases only        linearly. If the voltage across the plates is reduced during        approach, the electrostatic force will also be reduced. If this        is precisely controlled, the impact energy of the plates can be        reduced. The precise control of the charge on the plates is        difficult, and has only been demonstrated on devices with        integrated CMOS drivers.    -   2) Waveform Control. Similar to method (1) the charge on the        parallel plates in this case is controlled empirically through        an arbitrary voltage waveform generator. This provides less        precise control compared to (1), but is often easier to        implement.    -   3) Non-linear springs. In this case the mechanics of the spring        do not follow a simple Hooke's Law behavior. Generally the        non-linearity is cubic and therefore provides too much        compensation, which then requires a much higher actuation        voltage, which can lead to breakdown or higher costs.

Accordingly, since each of these methods is difficult to implementand/or involves additional cost, a parallel plate MEMS device is neededthat does not exhibit the runaway instability.

SUMMARY

We describe here a device that does not exhibit the runaway instabilityand thus provides for a much more controlled and gentle impact of thesurfaces. The device is described with respect to a particularembodiment, which is a microfabricated (MEMS) plate switch. The plateswitch may be initially separated by an initial distance or gap, D1,before actuation forces are applied.

The device may be mechanically constrained to operate within the top ⅓of its initial distance, D1. The mechanical constraint may be a post orraised feature fabricated on at least one of the surfaces. Because ofthe mechanical constraints, the surfaces can be made to impact atessential zero velocity. This is done such that no penalty is realizedin the loading force of the surfaces when in contact. This also allowsfor a broad range of flexibility in the applied waveform.

The device may be fabricated with one or more raised features on atleast one of the surfaces such that the surfaces interfere when the gapis less than or equal to ⅔ of the gap height. These features may be madeusing MEMS lithographic techniques, as described below. For this device,the one or more raised features will touch before the runaway conditionis reached. Accordingly, the device may include a first plate separatedfrom a second plate by an initial distance, wherein at least one of thefirst plate and the second plate further comprises a raised feature,wherein the raised feature limits the gap between the surfaces to about66% of the initial distance when an actuation is applied.

The device may be, for example, a parallel plate MEMS device which isactuated by a force. The actuation force may arise from any of a numberof phenomena, including electrostatic, magnetostatic, electromagnetic orpiezoelectric effects. In other embodiments, the MEMS device may be atleast one of an actuator, a switch and a sensor.

Accordingly, more generally, a device is described which may include afirst surface separated from a second surface by an initial distance D1,wherein at least one of the first surface and the second surface furthercomprises one or more raised features of height D2, wherein the one ormore raised features limits a gap between the surfaces to the distanceD2, wherein D2<D1, when an actuation is applied. Furthermore, in atleast one embodiment, the distance D2 may be about 66% of D1.

These and other features and advantages are described in, or areapparent from, the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary details are described with reference to the followingfigures, wherein:

FIG. 1 is a plot showing the separation of a hypothetical device;

FIG. 2 is a schematic diagram of a capacitor on a spring;

FIG. 3 is a schematic diagram of the inventive switch; and

FIG. 4 is a plot of simulation results for the inventive switch; and

FIG. 5 is a cross sectional view of an exemplary MEMS switch device witha separation limiting standoff;

It should be understood that the drawings are not necessarily to scale,and that like numbers may refer to like features.

DETAILED DESCRIPTION

The device described herein is mechanically constrained to operatewithin the top ⅓ of the initial gap. The mechanical constraint may be araised feature fabricated on at least one of the surfaces. The raisedfeature may have any arbitrary shape, such as a post, a bump, a sphere,a pyramid, a trapezoid, but in any case, the feature has a surface thatprotrudes beyond the adjacent, relatively flat surface. Furthermore, theraised feature may be one of a plurality of raised features, and theraised feature(s) may be placed uniformly around a plate, or near or atthe periphery of the plate, or in a single location, for example. Thenumber and placement of the raised feature(s) will depend on the detailsof the application and the architecture of the MEMS device.

Because of the mechanical constraint provided by the raised feature, thesurfaces can be made to impact at essentially zero velocity. This isdone such that no penalty is realized in the loading force of thesurfaces when in contact. This also allows for a broad range offlexibility in the applied waveform. One embodiment of the concept is aparallel plate electrostatic MEMS switch. However, It should beunderstood that the concept may be applied to other sorts of devices andother sorts of actuation mechanisms.

In one exemplary embodiment, the actuation mechanism may beelectrostatic, with an attractive force arising between the surfaces asa result of an applied voltage. However, this is but one example, andother actuation mechanisms may be used, including magnetic,magnetostatic, electromagnetic and piezoelectric, for example.

In one embodiment, the device employs a raised feature on one or both ofthe plates in a parallel-plate electrostatic MEMS device. This featurecomes into contact with the opposing plate before the applied voltagecan displace the plate more than ⅓ of the V=0 separation. Referring tothe FIG. 1, the V=0 separation of this hypothetical device is 1.6 um. Ifthe parallel-plate voltage does not exceed 32V, the plates will notimpact one another. At or below this voltage, the plates will undergo adisplacement of no more than ˜0.51 um, or ˜⅓ of 1.6 um, and thedisplacement is a monotonic function of the applied voltage.

The implementation of this can be explained by considering the“capacitor on a spring” concept. This is shown schematically in FIG. 2.

If the top plate is grounded and a voltage V is applied to the bottomplate, the top plate will move down until the spring force, which pullsup, is equal but opposite to the electrostatic force, which pulls down.Once the top surface moves ⅓ of the gap distance, however, aninfinitesimal increase in the applied voltage will cause the top beam tosnap down against the bottom surface at high velocity.

The high velocity runaway condition can be avoided using theconfiguration shown in FIG. 3. As illustrated in FIG. 3, an uppersurface 10 and a lower surface 30 are separated by an initial distanceD1. The upper surface 10 has a post 20 of height D2 fabricated thereon.When the actuation voltage is applied from a power supply (not shown)the switch will close to a minimum separation of D2, wherein D2 is lessthan D1. In some embodiments, D2 is about 66% of D1.

Any number of raised features may be created, each with a height that isgreater than or equal to ⅔ of the initial distance. In any case, theraised feature will touch down on the opposite surface before therunaway condition is reached. While in one embodiment, a post is used asthe raised feature, it should be understood that any of a number ofalternative or arbitrary shapes may be used, as long as the feature ismechanically competent to withstand the forces and define the minimumseparation between the surfaces.

Many suitable dielectric materials exist for the raised feature. Theraised feature may comprise, for example, an inorganic dielectric layersuch as silicon dioxide, silicon nitride, polysilicon, amorphoussilicon, spin-on glass (SOG), or a spin coated, temperature tolerantpolymer layer such as SU8, polyimide, or benzocyclobutene (BCB). Thesematerials may be formed or deposited in arbitrary shapes using knownlithographic techniques.

The effect of the raised feature is evident in the plot shown in FIG. 4,which shows simulation results of the displacement (x) and velocity (v)of the surface as a function of time (sec). Here a step function voltageis applied to the bottom surface with the top surface grounded. At thepoint where raised feature comes into contact (where the top surface hasmoved 1 um), the velocity of the surface is roughly 1 mm/sec. In theabsence of the raised feature the top surface would continue to moveuntil it hits the bottom surface (3 um in this simulation). Here thevelocity is >20 mm/sec. Thus, the impact energy in the case without theraised feature is 400× that with the raised feature, thus leading toimproved reliability.

The raised feature may be disposed on a surface of an electrostaticplate switch, which delivers an RF input signal to a set of output pads.The switch may have an operating frequency of at least about 1 MHz, suchthat when the switch is closed, the input signal is capacitivelydelivered to the output pads. More specifically, the capacitive plateswitch may form a nominally closed, capacitive connection when the twoplates are separated by the distance D2 and form an nominally openconnection when separated by the distance D1, for an RF signal at orabove an operating frequency. In the nominally closed position, theinput signal is delivered to the output pads; in the nominally openposition, it is not. In some embodiments, the operating frequency may beat least about 1 MHz.

FIG. 5 is a cross sectional view of an embodiment of the MEMSelectrostatic switch device 100 with a separation limiting standoff1100. This embodiment may be fabricated on two substrates, a platesubstrate 1000 and a via substrate 2000. The plate substrate 1000 may bean SOI wafer, and the via substrate may be a silicon wafer, for example.The SOI plate substrate 1000 may include a silicon device layer 1010, aninsulating layer 1020, and a thicker, silicon handle layer 1030. SOIwafers are well known in the art.

The switch 100 may include a plate 1300 bearing at least one standoff1100. The standoff 1100 may include an insulating pad 1150 and amechanically competent separation limiting feature 1160. The plate 1300may be deformable, meaning that it is sufficiently thin compared to itslength or its width to be deflected when a force is applied, and mayvibrate in response to an impact. For example, a deformable plate maydeflect by at least about 10 nm at its center by a force of about 1μNewton applied at the center, and sufficiently elastic to supportvibration in a plurality of vibrational modes. The deformable plate 1300may be suspended above the handle layer 1030 of an SOI plate substrate1000 by four spring beams (not shown in FIG. 5), which are themselvesaffixed to the silicon handle layer 1030 by anchor points formed fromthe insulating dielectric layer 1020 of the SOI plate substrate 1000. Asused herein, the term “spring beam” should be understood to mean a beamof flexible material affixed to a substrate at a proximal end, andformed in substantially one plane, but configured to move and provide arestoring force in a direction substantially perpendicular to thatplane. The deformable plate 1300 may carry at least one conductive shuntbar which operate to close the switch 100, as described below.

Additional details of such a device are disclosed in U.S. Pat. No.7,893,798 B2, issued Feb. 22, 2011 and assigned to the same assignee.This patent is incorporated by reference in its entirety. Unlike the'798 patent however, there is no electrical shunt bar in the embodimentshown in FIG. 5, and the movement of the parallel plates results only ina change of capacitance between the plates, rather than an electricalconnection between them.

The deformable plate 1300 may be actuated electrostatically by anadjacent electrostatic electrode 2300, which may be disposed directlyabove (or below) the deformable plate 1300, and may be fabricated on thevia substrate 2000. The deformable plate 1300 itself may form one plateof a parallel plate capacitor, with the electrostatic electrode 2300forming the other plate. When a differential voltage is placed on thedeformable plate 1300 relative to the adjacent electrostatic electrode2300, the deformable plate is drawn toward the adjacent electrostaticelectrode 2300. The action raises (or lowers) the separation limitingstandoff 1100 into a position where it contacts the contact points 2112and 2122, thereby capacitively closing an electrical circuit. Althoughthe embodiment illustrated in FIG. 5 shows the plate formed on the lowersubstrate and the vias and contacts formed on the upper substrate, itshould be understood that the designation “upper” and “lower” isarbitrary. The deformable plate may be formed on either the uppersubstrate or lower substrate, and the vias and contacts formed on theother substrate. However, for the purposes of the description whichfollows, the embodiment shown in FIG. 1 is presented as an example,wherein the plate is formed on the lower substrate and is pulled upwardby the adjacent electrode formed on the upper substrate.

The MEMS electrostatic switch device 100 with a separation limitingstandoff 1100 may be fabricated as follows. Beginning with the platesubstrate 1000, an insulating layer of dielectric material 1020, such asSiO₂ may be grown or deposited on the silicon surfaces. Alternatively,the SiO₂ layer may exist as the insulating layer on asilicon-on-insulator (SOI) substrate 1000. The dielectric layer 1020 maythen be etched away beneath and around the deformable plate 1300, usinga hydrofluoric acid liquid etchant, for example. The liquid etch mayremove the silicon dioxide dielectric layer 1020 in all areas where thedeformable plate 1300 is to be formed. The liquid etch may be timed, toavoid etching areas that are required to affix the spring beams of thedeformable plate 1300, which will be formed later, to the handle layer1030. Additional details as to the dry and liquid etching procedure usedin this method may be found in U.S. patent application Ser. No.11/359,558 (Attorney Docket No. IMT-SOI Release), filed Feb. 23, 2006and incorporated by reference in its entirety.

The next step may be the formation of the dielectric pads 1150 anddielectric standoffs 1160 as depicted in FIG. 5. Pad structures 1150 mayform an electrical isolation barrier between the standoff 1160 and thedeformable plate 1300, functioning as was described above. Thedeformable plate 1300 and adjacent actuation electrode 2300 form the twoplates of a parallel plate capacitor, such that a force exists betweenthe plates when a differential voltage is applied to them, drawing thedeformable plate 1300 towards the adjacent electrostatic electrode 2300.

The dielectric pad 1150 may be silicon dioxide, which may besputter-deposited or thermally grown over the surface of the devicelayer 1010 of the SOI plate substrate 1000. The silicon dioxide layermay be deposited to a depth of, for example, about 300 nm. The 300 nmlayer of silicon dioxide may then be covered with photoresist which isthen patterned. The silicon dioxide layer is then etched to formstructure 1150. The photoresist is then removed from the surface of thedevice layer 1010 of the SOI plate substrate 1000. Because thephotoresist patterning techniques are well known in the art, they arenot explicitly depicted or described in further detail.

Finally, a material is deposited and patterned to form the separationlimiting standoff 1160. The material may be any mechanically competentmaterial such as silicon nitride or photoresist. Since the pad layer isdielectric, metal may also be used for the separation limiting standoff.The conductive material may actually be a similar multilayer comprisingfirst a thin layer of chromium (Cr) for adhesion to the silicon and/orsilicon dioxide surfaces. The Cr layer may be from about 5 nm to about20 nm in thickness. The Cr layer may be followed by a thicker layerabout 300 nm to about 700 nm of gold (Au), as the conductivemetallization layer. Preferably, the Cr layer is about 15 nm thick, andthe gold layer is about 600 nm thick. Another thin layer of molybdenummay also be used between the chromium and the gold to prevent diffusionof the chromium into the gold, which might otherwise raise theresistivity of the gold. This layer may also participate in the bondingof the substrates.

More generally, the dielectric separation limiting feature may be atleast one of silicon dioxide, silicon nitride, polysilicon, amorphoussilicon, spin-on glass (SOG), or a spin coated, temperature tolerantpolymer layer such as SU8, polyimide, or benzocyclobutene (BCB).

Turning now to the via substrate 2000, another metallization region maybe deposited over the substrate 2000, as shown in FIG. 5. Thismetallization layer may form the bond ring 2400 as well as adjacentelectrostatic electrode 2300. The metallization region may define thesecond plate 2300 of the parallel plate capacitor of the switch. In oneexemplary embodiment, the metallization layer may actually be amultilayer of Cr/Au, the same multilayer as was used for themetallization layer 1400 on the plate substrate 1000 of the dualsubstrate electrostatic MEMS plate switch 100. The metallizationmultilayer may have similar thicknesses and may be deposited using asimilar process as that used to deposit metallization layer 1400 onsubstrate 1000. The metallization layer may also serve as a seed layerfor the deposition of a metal solder bonding material, as described inthe incorporated '798 patent. Layer 2200 may be a native insulatinglayer of SiO₂ that forms around the silicon substrate 2000. Two moreexternal (to the switch) electrical pads 2115 and 2125 may be connectedto through substrate vias 2110 and 2120 (TSV) may provide electricalaccess to the two electrical nodes 2112 and 2122 within the device 100.Layer 2200 may be a native insulating layer of SiO₂ that forms aroundthe silicon substrate 2000.

Each of the Cr and Au layers may be sputter-deposited using, forexample, an ion beam deposition chamber (IBD). The conductive materialmay be deposited in the region corresponding to the shunt bar 1100, andalso the regions which will correspond to the bond line 1400 between theplate substrate 1000 and the via substrate 2000 of the dual substrateelectrostatic MEMS plate switch 100. This bond line area 1400 ofmetallization will form, along with a layer of indium, a seal which willhermetically seal the plate substrate 1000 with the via substrate 2000.

To form the switch, SOI substrate 1000 is pressed against substrate 2000and the substrates are bonded together in a wafer bonding chamber forexample. The adhesive may be a thermocompression bond, a metal alloybond, or a glass frit bond for example. At bonding, thesubstrate-to-substrate separation is determined by a standoff 2400 inthe bondline, and this separation is approximately D1 as shown in theFigure.

The deformable plate 1300 on substrate 1000 and adjacent actuationelectrode 2300 on substrate 2000 may form the two plates of a parallelplate capacitor, such that a force exists between the plates when adifferential voltage is applied to them, drawing the deformable plate1300 towards the adjacent actuation electrode 2300.

A differential voltage may be applied to the deformable plate 1300 andthe second plate through another set of TSVs (not shown in FIG. 5). Uponapplication of a differential voltage between the first plate 1300 andthe second plate 2300, the two plates will be drawn together, but theirseparation limited by the standoff 1100. This minimum separation isdefined by the standoff 1100 to be about D2. As before, D2 is about 66%of D1.

At these small separations between the conductors, an RF signaldelivered to electrical node 2112 may be capacitively coupled to theother node 2122. Accordingly, there may be no electrical connectionbetween the first plate 1300 and the second plate 2300 or between nodes2112 and 2122. The actuation of the electrodes 1300 and 2300 maytherefore increase the capacitance of the switch, thereby “closing” acapacitive switch, delivering a signal from one electrical node 2112 tothe other 2122.

In another embodiment, another dielectric material may be placed on oneor the other of both conductive surfaces 1300 and 2300. The thickness ofthis dielectric layer may be less than the height of the separationlimiting standoff 1100. The purpose of the additional dielectricmaterial is to increase the capacitance of the switch due to thedifference in dielectric constant between the dielectric material andgas or vacuum. In this embodiment, the separation limiting standoffwould reduce the contact area of the plates and thus reduce the chargingor stiction.

In this embodiment, the additional dielectric material may again be anoxide such as silicon dioxide, or it may be a spun on or depositedglass, photoresist or ceramic.

The total thickness of the separation limiting feature may be on theorder of tens to hundreds of nanometers, and may depend on the geometryof the plates, for example, and the tolerances of the manufacturingprocesses. The thickness of the additional dielectric material may be onthe order of tens of microns, may again, depend on the details of theapplication.

While various details have been described in conjunction with theexemplary implementations outlined above, various alternatives,modifications, variations, improvements, and/or substantial equivalents,whether known or that are or may be presently unforeseen, may becomeapparent upon reviewing the foregoing disclosure.

What is claimed is:
 1. A device comprising: a first surface separatedfrom a second surface by an initial distance D1, wherein at least one ofthe first surface and the second surface further comprises one or moreseparation limiting standoffs of height D2, wherein the one or moreseparation limiting standoffs limits a gap between the surfaces to thedistance D2, wherein D2<D1, when an actuation is applied.
 2. The deviceof claim 1, wherein D2 is about 66% of D1.
 3. The device of claim 1,wherein the device is at least one of an actuator, a switch and asensor.
 4. The device of claim 1, wherein the actuation is at least oneof electrostatic, magnetostatic, electromagnetic or piezoelectric. 5.The device of claim 1, wherein the device is a capacitive plate switch,which forms a closed, capacitive connection when the two plates areseparated by the distance D2 and forms an open connection when separatedby the distance D1, for an RF signal at or above an operating frequency.6. The device of claim 5, wherein the operating frequency is at least 1MHz.
 7. The device of claim 1, wherein the one or more separationlimiting standoffs is a feature in the shape of at least one of a post,a bump, a sphere, a pyramid, a trapezoid.
 8. The device of claim 1,wherein the one or more separation limiting standoffs is one in aplurality of raised features.
 9. The device of claim 1, wherein the oneor more separation limiting standoffs are disposed near or at theperiphery of the surfaces.
 10. The device of claim 1, wherein the one ormore separation limiting standoffs comprise a dielectric substancechosen from the group consisting of silicon dioxide, silicon nitride,polysilicon, amorphous silicon, spin-on glass (SOG), or a spin coated,temperature tolerant polymer layer such as SU8, polyimide, orbenzocyclobutene (BCB).
 11. The device of claim 1, wherein the device isa MEMS electrostatic plate switch, having two substantially parallelplates.
 12. The device of claim 11, wherein the two plates move towardone another upon application of a differential voltage between them. 13.The device of claim 12, wherein the two substantially parallel platesact to open and close a capacitive switch, by changing a capacitancebetween them as a result of their movement upon application of thedifferential voltage.
 14. The device of claim 13, wherein the two platesare formed on two difference subtrates.
 15. The device of claim 14,wherein at least one of the substrates is an SOI wafer.
 16. The deviceof claim 15, wherein the movement of the plates is constrained by theseparation limiting standoff to a value of closest approach equallingD2.
 17. The device of claim 16, wherein the separation limiting standoffcomprises an insulating pad and a mechanically competent feature. 18.The device of claim 17, wherein the insulating pad comprises at leastone of a metal oxide, a semiconductor oxide, a photoresist, a glass or aceramic.
 19. The device of claim 17, wherein the mechanically competentfeature comprises at least one of a metal, a metal oxide, a metal alloy,or an insulating material.
 20. The device of claim 18, wherein theinsulating pad comprises silicon dioxide.